Prior art automatic drinking water supply systems for animals typically employ a contact type water flow control means such as a float valve which is installed within or adjacent the water drinking trough. However float valves are prone to malfunction due to contact by the animal which may damage or dislodge the float valve causing the valve to stick in either an open or closed position. When stuck in an open position, the water trough may overflow resulting in a waste of water and formation of mud adjacent the water trough. And, if the float valve is stuck in a closed position, the animal may become dehydrated. For large animals such as horses and cattle who might consume 5 to 10 gallons of water, or more, daily, particularly in warmer climates, maintaining an adequate supply of drinking water is imperative for the animal's health.